class_docstr: Generic data structure representing a directed graph with multiple edges.
docstr: Add an edge from `from_key` to `to_key`.
docstr_tok: ["Add","an","edge","from","`","from_key","`","to","`","to_key","`","."]
code: def add_edge(self, from_key: KT, to_key: KT, value: VT) -> None:
      """Add an edge from `from_key` to `to_key`."""
      if from_key not in self._adjacency_dict:
          self._adjacency_dict[from_key] = {}
      if to_key not in self._adjacency_dict[from_key]:
          self._adjacency_dict[from_key][to_key] = []
      self._adjacency_dict[from_key][to_key].append(value)
  
file_path: metricflow/metricflow/model/graph.py
imports: from __future__ import annotations
  from typing import Dict, Generic, List, Tuple, TypeVar
label_desc: Use image node_id 5 for calling the MultiDiGraph obj's underlying member method code with example usage: obj.add_edge(from_key, to_key, value) without return types